1. Field of the Invention
The present invention relates generally to recording apparatuses for recording information such as characters and images, and also the present invention relates to information-processing systems using such apparatuses as output devices, such as facsimiles, printers, word-processors, and personal computers. The term "recording" refers to all kinds of procedures for providing ink on ink supports such as cloth, yarn, paper, and other sheet materials (i.e., printing process, image-forming process, copying process, dye or staining process, and the like). Therefore the field of the invention is not limited to the field of information-processing, and so it is also applicable to other fields including apparel industries using the ink supports for receiving ink such as cloth, yarn, paper, and other sheet materials.
2. Description of the Related Art
Heretofore, ink-jet recording apparatuses have been used as output means on the commercial basis, for example they have been used as printers as output terminals of copying machines, facsimiles, electronic typewriters, word processors, work stations, and the like, and also such as handy- or portable-type printers equipped in personal computers, host computers, optical disk apparatuses, video apparatuses, and so on. In these cases, each of the ink-jet recording apparatuses is configured to meet a specific function, a usage pattern, and the like of the corresponding apparatus.
In general, the ink-jet recording apparatus comprises a carriage on which a recording means (a recording head) and an ink tank are mounted, a paper-feed means for feeding a sheet of recording paper, and a control means for controlling movements of these means. The recording head for ejecting ink droplets from a plurality of orifices thereof is driven to sequentially scan over a surface of the recording medium (i.e., serial-scanning movement) in the direction (i.e., main-scanning directions perpendicular to the feeding direction (i.e., sub-scanning direction) of the recording paper at the period of a recording movement. In addition, the recording medium is intermittently shifted at a distance corresponding to a recording width of the recording medium at the period of a non-recording movement. In this way, therefore, the recording apparatus performs a recording movement by ejecting ink droplets from the recording head on the recording paper in accordance with recording signals and it shows an excellent cost/performance ratio, so that it has been widely used as one that performs the recording movement silently.
Furthermore, it is possible to perform a high-speed recording movement by using a recording head on which a plurality of nozzles for ejecting ink are arranged in a line along the sub-scanning direction because the recording head records an image with a width corresponding to the number of nozzles by carrying out the scanning movement one time.
In the case of an ink-jet recording apparatus for multiple-color recording, ink droplets can be ejected from plural recording heads that correspond to different colors, respectively, and so a color image can be formed by placing ink droplets one upon another. For the multiple-color recording, in general, four different types of the recording heads and the ink cartridges corresponding to three primary colors (i.e., yellow (Y), magenta (M), and cyan (C)) and/or black (B) are required. In recent years, by the way, recording apparatuses having three or four recording heads for full-color recording have been developed and used in practical applications.
For the conventional recording apparatuses described above, there are several kinds of means for generating energies to be used for ejecting ink, such as electromagnetic transducers (e.g., piezo elements) and electrothermal transducers having electric heating elements to heat a liquid. Among those means, the recording head that uses the means in the type (so-called bubble-jet type) of ejecting a liquid by utilizing heat energies (i.e., utilizing a membrane-boiling phenomenon) is possible to perform a high-resolution recording because the ejection orifices can be arranged at a high density.
The bubble-jet process for forming an ink droplet by the recording head constructed as described above will be described briefly in the followings.
First of all, a membranous air bubble is generated by the phenomenon of film boiling to such an extent as to cover a surface of a thermal resistor (hereinafter, referred as a heater) when the heater reaches to a predetermined temperature. An inner pressure of the bubble is extremely high, so that the bubble pushes ink in a nozzle out. Then the ink moves toward an outside of the nozzle and also toward a common liquid chamber in the opposite direction thereof by effective force as a result of the pushing out. While this action is in process, the inner pressure of the bubble becomes negative and the ink loses velocity in the nozzle as a result of causing a flow resistance in a path of flow in addition to the effective force. The ink ejected from the orifice to the outside flies faster than the speed of ink in the nozzle, so that the ink becomes constricted and a part thereof is separated to generate an ink droplet by an effect of a balance among the effective force, flow-path resistance, bubble shrinkage, and surface tension of the ink. Then the nozzle is supplied with additional ink from the common liquid chamber by an effect of capillary pressure generated concurrently with the bubble shrinkage, and then the nozzle waits the next pulse to be applied.
Accordingly, the ink-jet recording apparatus in particular attains a rapid-response ink-ejection by using the recording head that uses electrothermal converting elements as energy generating means because of generating the bubble in ink in a liquid path by a drive signal in the form of an electric pulse in a one-to-one relationship and of growing and contracting the bubble quickly and appropriately.
According to the above construction, the ink-jet recording head has advantages in its suitability for being made as compact as possible with a high board density and low manufacturing cost because it is possible to make more use of the advantages of IC technologies and micro-processing technologies with recent considerable improvements in reliability and with recent considerable progress in semiconductor technologies.
For providing a gradation by using the ink-jet recording head constructed as described above, the following methods have been known:
a method of representing the entire gradation of a picture by elements consisting one of two levels separated by a predetermined threshold, in which an image signal for one element corresponds to one of the two levels (i.e., a dither method); PA1 a method disclosed in, for example Japanese Patent Laying-open Nos. 207265/1984, 160654/1982, or 53052/1988, representing the entire gradation of a picture, having the steps of changing the size of each liquid droplet by combining plural liquid droplets together and placing the combined liquid droplet on a recording medium (i.e., a liquid droplet modulating method); and PA1 a method as disclosed in Japanese Patent Laying-open No. 53052/1988 including the steps of obtaining a gradation of a picture by changing the number of liquid droplets to be placed on a recording medium, where one dot is formed by placing plural liquid droplets on the same place of a recording medium (i.e., a multiple droplets method). PA1 preparing a means for ejecting inks of different densities in a plurality of ejection amounts, respectively; PA1 linearizing a relationship between gradation and image density so that levels of the gradation of each of the inks of different densities can be interpolated with others; and PA1 making a record on a recording medium in response to multiple-level recording data in accordance with the relationship between gray scale and image density. PA1 a plurality of image densities obtained by a plurality of types of inks having different ink densities; and PA1 at least one image density between two of the plurality of image densities, obtained by changing an ejection amount of the ink. PA1 means for ejecting inks of different densities in a plurality of ejection amounts, respectively; PA1 means for linearizing a relationship between gradation and image density so that levels of the gradation of each of the inks of different densities can be interpolated with others; and PA1 means for making a record on a recording medium in response to multiple-level recording data in accordance with the relationship between gray scale and image density. PA1 means for obtaining a plurality of image densities by a plurality of types of inks having different ink densities; and PA1 means for obtaining at least one image density between two of the plurality of image densities by changing an ejection amount of the ink.
In these methods, however, the dither method has a problem that a resulting image is of low resolution while the liquid droplet modulating method has a problem that it is difficult to obtain a wide range of the gradation so as to correspond to a practical range of a distance between the sheets of paper in the recording device. In the case of the multiple droplets method, on the other hand, it is far superior to the others because it makes possible to record an image at high resolution and multiple levels of gray by selecting an ink-jet recording head that ejects small sized liquid droplets.
Generally, the flow of ink through a recording medium requires several hundred meters per minute after placing an liquid droplet on the recording medium. As a result, a hemispherical droplet is formed on the recording medium when the liquid droplet is placed in a position in advance of ejecting other liquid droplets. A comparatively large sized hemispherical liquid droplet can be formed by placing a following liquid droplet on a preceding liquid droplet on the recording medium. In this case, a size of each pixel can be modified by changing the number of liquid droplets to be placed on the same place one after another.
For obtaining a broader range of the gray-sale than ever before, there is an idea of increasing the number of liquid droplets to be placed one after another and in this case a recording speed may be decreased. However, this kind of problem can be solved by the above method of modulating liquid droplets, a method including the steps of placing the liquid droplets of different volumes one after another on a recording medium by ejecting these droplets in a very short time from different nozzles of the ink-jet recording head disclosed in Japanese Patent Application Publication No. 502261/1988, and so on, and also a combination of these methods.
FIGS. 1A to 1E and FIG. 2 are diagrammatic sectional illustrations and a diagrammatic wave form, respectively, for explaining an example of pixel formation according to a so-called multiple droplets system by which one pixel can be formed by ejecting plural liquid droplets from one nozzle. In this system, a plurality of liquid droplets are substantially placed one after another on the same place of a recording medium (in this case, a sheet of recording paper). Therefore, it is also conceivable that one liquid droplet makes one pixel. In the figures, a pixel frequency f0 is responsible for determining a minimum distance between adjacent pixels in the main-scanning direction of an ink-jet recording head. Also, a frequency f of ejecting liquid droplets is responsible for determining the number of ink droplets to be placed substantially on the same place of the recording paper. In the following description, we will focus on one in a plurality of nozzles formed on the ink-jet recording head mounted on an ink-jet recording apparatus.
At first, an ejection signal P21 is impressed on a ejecting means arranged in a nozzle. Then the ejection means is driven to eject a liquid droplet D1 from the nozzle. The liquid droplet D1 forms a pixel 101 on a sheet of recording paper 103. After a lapse of 1/f hour from the time of impressing the ejection signal P21, an ejection signal P22 is selectively impressed on the ejecting means. Depending on the ejection signal P22, a liquid droplet D2 is ejected on the recording paper 103 so as to be placed over at least a part of the pixel 101 (for the reason that the ink-jet recording head moves for the distance corresponding to 1/f hour in the main-scanning direction). In the same way, plural liquid droplets complete the formation of a pixel 102. That is, the pixel 102 is formed on the pixel 101 by ejecting liquid droplets D3, D4 on the recording paper 103 as a result of impressing ejection signals P23, P24 on the ejecting means at predetermined intervals. In the above steps, a size of the pixel formed on the recording paper 103 can be changed in accordance with the presence or absence of impressing the ejection signals P21, P22, P23, and P24. In the case of forming another pixel so as to be adjacent to the pixel 102, a liquid droplet may be ejected by impressing an ejection signal P21 or the like after the expiration of a predetermined time interval (1/f0).
In the conventional ink-jet recording apparatuses as described above, each recording apparatus adopts a recording system in which one apparatus mounts only one recording head, so that a gradation is represented by varying the number of liquid droplets to be placed on the same place of a recording medium one after another by the recording head to form one dot. If more levels of the gradation are required, the number of liquid droplets to be ejected for one dot is increased and it results in a substantial drop in a recording speed.
If the design of periphery of nozzles is simple, the following technical challenges should be attained. That is, in the case of a recording apparatus for multiple-color recording plural recording heads cannot be driven to provide different width and levels of gradation in each of them, resulting that they are driven to provide the same width and levels of gradation on every occasion. Consequently, an unstable ejection behavior and an undesired gradation may be obtained because of the different ejection properties of inks depending on their types. The number of types of inks should be determined so as to correspond to the desired width or levels of gradation when it is represented by the difference in gradations in color or shades of gray depending on the types of ink.